The present invention relates to a heat transfer image-receiving sheet, and more particularly to a heat transfer image-receiving sheet capable of producing an image which is excellent in color density, sharpness and fastness properties, in particular, in resistance to light, resistance to sebum and sweat, resistance to plasticizer, resistance to oils and resistance to heat.
Heretofore, a variety of heat transfer printing methods have been known. One of them is a method in which a heat transfer sheet comprising as a recording agent a sublimable dye which is retained by a substrate sheet such as a polyester film, used in combination with an image-receiving sheet capable of being dyed with the sublimable dye, prepared by providing a dye-receiving layer on a substrate sheet such as paper or a plastic film to produce various full-colored images on the image-receiving sheet.
In the above method, a thermal head of a printer is employed as a heat application means, and a large number of dots in three or four colors are transferred to the image-receiving sheet in an extremely short heat application time. A full-colored original image can thus be successfully reproduced on the image-receiving sheet.
The image thus produced is excellent in sharpness and clarity because a dye is used as a coloring agent. Therefore, the heat transfer printing method of this type can produce an excellent half-tone image with continuous gradation, comparable to an image obtained by offset printing or gravure printing. Moreover, the quality of the image is as high as that of a full-colored photograph.
In the above heat transfer printing method, not only the structure of the heat transfer sheet but also that of the image-receiving sheet on which an image is produced is an important factor.
Conventional heat transfer image-receiving sheets disclosed, for instance, in Japanese Laid-Open Patent Publication Nos. 169370/1982, 207250/1982 and 25793/1985 comprise a dye-receiving layer which is formed using a resin selected from polyester resins, vinyl resins such as a polyvinyl chloride resin, polycarbonate resins, polyvinyl butyral resins, acrylic resins, cellulose resins, olefin resins and polystyrene resins.
The above heat transfer image-receiving sheets, however, are disadvantageous in that their dye-receiving layers are poor in dye-receptivity, and that images produced therein are insufficient in fastness properties and preservability. It is therefore required to find materials suitable for a dye-receiving layer which is free from all the above problems.
The use of a resin having high dye-receptivity or the incorporation of a plasticizer may be effective to form a dye-receiving layer having high dye-receptivity. This is because a dye thermally transferred to such a dye-receiving layer can easily diffuse therein. However, an image produced in the dye-receiving layer formed using a resin having high dye-receptivity tends to blur in the course of the preservation thereof. In other words, such a dye-receiving layer is poor in the preservability of images. Moreover, the dye cannot be well fixed in the dye-receiving layer, so that it tends to bleed on the surface of the dye-receiving layer. As a result, an object which is brought into contact with the dye-receiving layer is stained with the dye.
To solve the above problems, the dye-receiving layer may be formed using a resin which does not allow the dye to easily migrate in the dye-receiving layer. However, the dye-receiving layer formed using such a resin is poor in dye-receptivity and cannot produce a highly sharp image with a high optical density.
There are some other problems in the prior art. Light resistance of the dye transferred to the dye-receiving layer is insufficient. In the case where the image-recorded surface of the dye-receiving layer is touched with fingers, the image undergoes a change in color or the dye-receiving layer itself swells or cracks due to sweat and sebum deposited by the fingers (resistance to such sweat and sebum is hereinafter referred to as "resistance to fingerprint"). Furthermore, when the dye-receiving layer is brought into contact with an article containing a plasticizer such as a plastic eraser or a product of a soft vinyl chloride resin (ex. telephone cord), the dye tends to migrate to the article. In other words, the dye-receiving layer has the problem of low resistance to plasticizer.
A polyester resin is conventionally known as a resin capable of forming a dye-receiving layer which is excellent in the above-described dye-receptivity, dye-fixating ability, resistance to fingerprint and resistance to plasticizer.
However, the light resistance of an image produced in a dye-receiving layer formed using a polyester resin is inferior to that of an image produced in a dye-receiving layer formed using a polyvinyl butyral resin or a polycarbonate resin. Further, although resistances to fingerprint and to plasticizer (oils) of the image produced in the dye-receiving layer formed using a polyester resin are superior to those of the image produced in a dye-receiving layer formed using a polycarbonate resin, a polyvinyl butyral resin or the like, they are unsatisfactory. The resistances to light, to plasticizer and to fingerprint greatly depend on the chemical structure of a resin which is used for forming the dye-receiving layer.